Phase change device having two or more substantial amorphous regions in high resistance state

ABSTRACT

Memory devices are described herein along with method for operating the memory device. A memory cell as described herein includes a first electrode and a second electrode. The memory cell also comprises phase change material having first and second active regions arranged in series along an inter-electrode current path between the first and second electrode.

PARTIES TO A JOINT RESEARCH AGREEMENT

International Business Machines Corporation, a New York corporation, andMacronix International Corporation, Ltd., a Taiwan corporation, areparties to a Joint Research Agreement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high density memory devices based onphase change based memory materials, including chalcogenide basedmaterials and on other programmable resistive materials, and methods foroperating such devices.

2. Description of Related Art

Phase change based memory materials, like chalcogenide based materialsand similar materials, can be caused to change phase between anamorphous state and a crystalline state by application of electricalcurrent at levels suitable for implementation in integrated circuits.The generally amorphous state is characterized by higher electricalresistivity than the generally crystalline state, which can be readilysensed to indicate data. These properties have generated interest inusing programmable resistive material to form nonvolatile memorycircuits, which can be read and written with random access.

The change from the amorphous to the crystalline state is generally alower current operation. The change from crystalline to amorphous,referred to as reset herein, is generally a higher current operation,which includes a short high current density pulse to melt or breakdownthe crystalline structure, after which the phase change material coolsquickly, quenching the phase change process and allowing at least aportion of the phase change material to stabilize in the amorphousstate.

In phase change memory, data is stored by causing transitions in anactive region of the phase change material between amorphous andcrystalline states. FIG. 1 is a graph of memory cells having one of twostates (storing a single bit of data), a low resistance set (programmed)state 100 and a high resistance reset (erased) state 102 each havingnon-overlapping resistance ranges.

The difference between the highest resistance R₁ of the low resistanceset state 100 and the lowest resistance R₂ of the high resistance resetstate 102 defines a read margin 101 used to distinguish cells in the setstate 100 from those in the reset state 102. The data stored in a memorycell can be determined by determining whether the memory cell has aresistance corresponding to the low resistance state 100 or to the highresistance state 102, for example by measuring whether the resistance ofthe memory cell is above or below a threshold resistance value R_(SA)103 within the read margin 101.

In order to reliably distinguish between the reset state 102 and the setstate 100, it is important to maintain a relatively large read margin101. However, it has been observed that some phase change memory cellsin the reset state 102 will experience an erratic “tailing bit” effectin which the resistance of the memory cell decreases over time to belowthe threshold resistance value R_(SA) 103, resulting in data retentionproblems and bit errors for those memory cells. Error Correction Coding(ECC) may be used to address the tailing bit issues, but may result inpenalties regarding write efficiency, read speed, and chip size.

It is desirable therefore to provide a memory cell structure and methodsfor operating such structures which address these data retention issueswithout ECC and result in improved data storage performance.

SUMMARY OF THE INVENTION

A memory device as described herein includes a memory cell comprising afirst electrode, a second electrode, and phase change material havingfirst and second active regions arranged in series along aninter-electrode current path between the first electrode and the secondelectrode. The memory device includes bias circuitry adapted to applybias arrangements to the memory cell to store a bit. The biasarrangements include a first bias arrangement adapted to establish ahigh resistance state in the memory cell by inducing a high resistancecondition in both the first and second active regions to store a firstvalue of the bit in the memory cell. The high resistance state has aminimum resistance indicating that at least one of the active regions isin the high resistance condition. The bias arrangements also include asecond bias arrangement adapted to establish a low resistance state inthe memory cell by inducing a low resistance condition in both of thefirst and second active regions to store a second value of the bit inthe memory cell. The low resistance state has a maximum resistanceindicating that both the first and second active regions are in the lowresistance condition. The memory device also includes sense circuitry tosense the value of the bit in the memory cell by determining if thememory cell has a resistance corresponding to the low resistance stateor to the high resistance state.

A method is described herein for operating a memory cell comprising afirst electrode, a second electrode, and phase change material havingfirst and second active regions arranged in series along aninter-electrode current path between the first and second electrode. Themethod includes determining a data value of a bit to be stored in thememory cell. A first bias arrangement is applied to the memory cell ifthe data value is a first value, the first bias arrangement adapted toestablish a high resistance state in the memory cell by inducing a highresistance condition in both the first and second active regions tostore the first value of the bit. The high resistance state having aminimum resistance indicating that at least one of the active regions isin the high resistance condition. A second bias arrangement is appliedif the data value is a second value, the second bias arrangement adaptedto establish a low resistance state in the memory cell by inducing a lowresistance condition in both the first and second active regions tostore the second value of the bit. The method includes determining thedata value of the bit stored in the memory cell by determining whetherthe memory cell has a resistance corresponding to the low resistancestate or to the high resistance state.

Since the high resistance state of the memory cell has a minimumresistance indicating that at least one of the active regions is in thehigh resistance condition, the present invention allows the data valuestored to be determined even if one of the active regions experiencesthe tailing bit effect. Thus, the tailing bit failure rate of an arrayof memory cells can be significantly reduced without the use of ECC.

Other aspects and advantages of the present invention can be seen onreview of the drawings, the detailed description, and the claims whichfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical distribution of memory cells having one of twostates, a low resistance set state and a high resistance reset state.

FIGS. 2A-2C illustrate schematic diagrams of three prior art phasechange memory cells having a phase change material memory element with asingle active region.

FIGS. 3A-3E illustrate cross-sectional views of prior art configurationsfor the memory element.

FIGS. 4-5 illustrate a possible early fail model for the tailing biteffect of memory cells in reset.

FIG. 6 is a graph of measured resistance data of memory cells in thereset state further illustrating the tailing bit effect.

FIG. 7 is a three-dimensional contour plot showing the distribution ofthe resistance of the memory cells after the first baking and the secondbaking.

FIG. 8A-8B illustrate the resistance distributions further demonstratingthe randomness of the tailing bit effect.

FIG. 9 is a map of the array with the dots on the map representing thelocation of the memory cells which experienced a difference in the ratioof the resistance after the first baking and the second baking ofgreater than 10.

FIGS. 10A-10B illustrate schematic diagrams of memory cells having twoactive regions arranged in series with a switching device.

FIG. 11A illustrates a method for programming a memory cell having aplurality of active regions arranged in series along an inter-electrodepath.

FIG. 11B illustrates a method for sensing a single bit stored in aselected memory cell having a plurality of active regions arranged inseries along an inter-electrode current path.

FIG. 12 is a simplified block diagram of an integrated circuit includinga memory array implemented using memory cells adapted to store one bitof data and having two or more active regions arranged in series.

FIG. 13 illustrates a portion of the memory array.

FIG. 14 illustrates the measured resistance distribution of an array of1T1R memory cells and the computed resistance distribution of an arrayof 1T2R memory cells.

FIGS. 15A-15B further illustrate a map of an array to further illustratethe guarding effect of the second active region.

FIG. 16 illustrates a graph of the failure rate of cells in an arrayversus time for various temperatures for both 1T1R and 1T2R memorycells.

FIG. 17 illustrates a cross-sectional view of a first memory cell havingfirst and second active regions arranged in series along aninter-electrode current path between first and second electrodes.

FIG. 18 illustrates a cross-sectional view of a second memory cellhaving first and second active regions arranged in series along aninter-electrode current path between first and second electrodes.

FIG. 19 illustrates a cross-sectional view of a third memory cell havingfirst and second active regions arranged in series along aninter-electrode current path between first and second electrodes.

FIG. 20 illustrates a cross-sectional view of a fourth memory cellhaving first and second active regions arranged in series along aninter-electrode current path between first and second electrodes.

FIG. 21 illustrates a cross-sectional view of a fifth memory cell havingfirst and second active regions arranged in series along aninter-electrode current path between first and second electrodes.

FIG. 22 illustrates a cross-sectional view of a sixth memory cell havingfirst and second active regions arranged in series along aninter-electrode current path between first and second electrodes.

DETAILED DESCRIPTION

The following description of the invention will refer to specificstructural embodiments and methods. It is understood that there is nointention to limit the invention to the specifically disclosedembodiments and methods but that the invention may be practiced usingother features, elements, methods, and embodiments. Preferredembodiments are described to illustrate the present invention, not tolimit its scope. Those of ordinary skill in the art will recognize avariety of equivalent variations on the description that follows. Likeelements in various embodiments are commonly referred to with likereference numerals.

FIGS. 2A-2C illustrate schematic diagrams of three prior art phasechange memory cells each having a phase change material memory element220 with a single active region (represented in the Figures by avariable resistor) and coupled to a select device such as a transistoror diode.

FIG. 2A illustrates a schematic diagram of a prior art memory cell 200including a field effect transistor (FET) 210 as the select device. Aword line 240 extending in a first direction is coupled to the gate ofthe FET 210 and a memory element 220 couples the drain of the FET 210 toa bit line 230 extending in a second direction.

FIG. 2B illustrates a schematic diagram of memory cell 202 similar tothat of FIG. 2A except that the access device is implemented as abipolar junction transistor (BJT) 212, while FIG. 2C illustrates aschematic diagram of a memory cell 204 similar to that of FIG. 2A exceptthe access device is implemented as a diode 214.

Reading or writing can be achieved by applying suitable voltages to theword line 240 and bit line 230 to induce a current through the memoryelement 220. The level and duration of the voltages applied is dependentupon the operation performed, e.g. a reading operation or a writingoperation.

FIGS. 3A-3E illustrate cross-sectional views of prior art configurationsfor memory element 220.

FIG. 3A is a simplified cross-sectional view illustrating a firstconfiguration for memory element 220 coupled to first and secondelectrodes 312, 314. The first electrode 312 may, for example, becoupled to a terminal of an access device such as a diode or transistor,while the second electrode 314 may be coupled to a bit line.

A dielectric spacer 313 having a width 315 separates the first andsecond electrodes 312, 314. The phase change material of memory element220 extends across the dielectric spacer 313 and contacts the first andsecond electrodes 312, 314, thereby defining an inter-electrode pathbetween the first and second electrodes 312, 314 having a path lengthdefined by the width 315 of the dielectric spacer 313. In operation, ascurrent passes between the first and second electrodes 312, 314 andthrough the memory element 220, the active region 318 of the phasechange material of the memory element 220 heats up more quickly than theremainder of the memory element 220.

FIG. 3B is a simplified cross-sectional view illustrating a secondconfiguration for memory element 220 coupled to first and secondelectrodes 322, 324. The phase change material of the memory element 220has an active region 328 and contacts the first and second electrodes322, 324 at top and bottom surfaces 323, 329 respectively. The memoryelement 220 has a width 321 the same as that of the first and secondelectrodes 322, 324.

FIG. 3C is a simplified cross-sectional view illustrating a thirdconfiguration for memory element 220 coupled to first and secondelectrodes 332, 334, the phase change material of memory element 220having an active region 338. The first and second electrodes 332, 334are separated by dielectric spacer 335. The first and second electrodes332, 334 and the dielectric spacer 335 have a sidewall surface 331. Thephase change material of memory element 220 is on the sidewall surface331 and extends across the dielectric spacer 335 to contact the firstand second electrodes 332, 334.

FIG. 3D is a simplified cross-sectional view illustrating a fourthconfiguration for memory element 220 coupled to first and secondelectrodes 342, 344. The phase change material of memory element 220 hasan active region 348 and contacts the first and second electrodes 342,344 at top and bottom surfaces 343, 349 respectively. The memory element220 has a width 341 less than that of the first and second electrodes342, 344.

FIG. 3E is a simplified cross-sectional view illustrating a fifthconfiguration for memory element 220 coupled to first and secondelectrodes 354, 352. The first electrode 354 has a width 351 less thanwidth 353 of the second electrode 352 and memory element 220. Because ofthe difference between width 351 and width 353, in operation the currentdensity in the phase change material of memory element 220 is largest inthe region adjacent the first electrode 354, resulting in the activeregion 358 having a “mushroom” shape as shown in the Figure.

As was described above, in an array some of the memory cells in the highresistance reset state will experience a tailing bit effect in whichthose memory cells undergo a reduction in resistance, resulting in dataretention issues and bit errors.

Illustrated in FIGS. 4 and 5 is a possible early fail model for thetrailing bit effect of memory cells in reset. Since the initial resetresistance of memory cells experiencing the tailing bit effect is high,a small or otherwise defective active region is not believed to be thelikely cause. Instead, in the early fail model illustrated in FIGS. 4and 5 a random distribution of crystallized regions within the generallyamorphous active region will undergo growth during baking. For memorycells experiencing the tailing bit effect the random arrangement of thecrystallized regions results in the need for very little growth before alow resistance path through the active region is formed.

FIG. 4A illustrates a “mushroom type” memory cell 400 having top andbottom electrodes 412, 414 and a memory element 416 comprising phasechange material. In reset the memory element 416 has a generallyamorphous active region 410 and a random distribution of crystallineregions 420 within the active region 410. As shown in FIG. 4B, afterbaking the crystalline regions 420 within the active region 410 willexperience growth but do not form a complete low resistance path throughthe active region 410. Thus, although the memory cell illustrated inFIGS. 4A and 4B may experience some reduction in resistance, it does notexperience the tailing bit effect.

FIGS. 5A and 5B illustrate a memory cell 500 having a randomdistribution of crystalline regions 520 within the active region 510such that after baking a low resistance path 550 is formed through theactive region 510 as shown in FIG. 5B, resulting in the memory cell ofFIGS. 5A and 5B experiencing the tailing bit effect.

FIG. 6 is a graph of measured resistance data of memory cells in thehigh resistance reset state further illustrating the tailing bit effect.The memory cells in the reset state initially have a resistancedistribution as given by the curve labeled “Before 1st baking”. As canbe seen in the Figure, baking at 130° C. results in some of the memorycells experiencing the tailing bit effect, with increased baking timecausing an increase in the number of memory cells at the low resistancetail (the left most portion of the curve) of the resistancedistribution.

After the first baking of the memory cells a reset operation isperformed to reset the memory cells to the high resistance state, theresulting resistance distribution of the cells as shown by the curvelabeled “Before 2nd baking” and being substantially the same as thecurve “Before 1st Baking”. As can be seen in the Figure, a second bakingat 130° C. results in a very similar number of memory cells experiencingthe tailing bit effect as the first baking, with increased baking timeagain causing an increase in the number of memory cells at the lowresistance tail of the resistance distribution.

As shown in FIG. 6, the resistance distributions of the cells after thefirst and second baking for each of the various baking times are verysimilar. However, as described in more detail below with reference toFIGS. 7 and 8, it has been observed that the cells which experience thetailing bit effect during the first baking are not necessarily the samecells which experience the tailing bit effect during the second baking,indicating a randomness as to whether or not a given memory cell willexperience the tailing bit effect.

FIG. 7 is a three-dimensional contour plot showing the distribution ofthe resistance of the memory cells after the first baking and the secondbaking of 10 hours at 130° C. respectively.

After the first baking the memory cells were assigned a first resistanceindex Ridx based on the measured resistance of the cell, with each firstresistance index Ridx being associated with one non-overlappingresistance range and arranged in order from lowest to highestresistance.

After the reset operation and the second baking, the resistance of eachof the memory cells were again measured and assigned a second resistanceindex Ridy based on the resistance of the cell after the second baking,where each second resistance index Ridy is associated with the sameresistance range as the corresponding Ridx (for example, Ridx=20 andRidy=20 are associated with the same resistance range).

As can be seen in the Figure, even though each of the memory cells in agiven Ridx had a resistance within the associated resistance range afterthe first baking, those same memory cells had a resistance distributionover a range of Ridy's after the second baking.

The range of Ridy's for the memory cells assigned a given Ridxdemonstrates the apparent randomness of the trailing bit effect. Forexample, some of the memory cells having a low Ridx have a high Ridywhich indicates that some memory cells which experienced the trailingbit effect in the first baking do not experience it in the secondbaking. Additionally, some of the memory cells having a high Ridx have alow Ridy which indicates that some of the memory cells which did notexperience the trailing bit effect in the first baking do experience itin the second baking.

The randomness of the tailing bit effect is further illustrated in FIG.8A, showing the resistance distribution 800 of the memory cells of the512 Kb array arranged by first resistance index Ridx after the firstbaking at 130° C. for 10 hours.

FIG. 8A also includes the Ridy resistance distribution 810 after thesecond baking of the 88,221 memory cells having a first resistance indexof Ridx=20. As can be seen, the distribution 810 shows that even thoughall of the 88,221 memory cells had a resistance after the first bakingwithin the resistance range associated with Ridx=20 (and thus none ofthe cells experienced the trailing bit effect), after the second bakingthose same 88,221 memory cells had a resistance distribution 810 overthe range of at least Ridy=10 to Ridy=21, including some memory cellswhich experienced the trailing bit effect.

Also shown in FIG. 8A is the predicted probability distribution 820 of88,221 memory cells after the second baking using only the distribution800 of the 512 Kb array. As can be seen in the Figure the predicteddistribution 820 matches the actual distribution 810 of Ridx=20,demonstrating that the resistance distribution after baking can beaccurately predicted based on previous distributions. This alsodemonstrates the randomness of the trailing bit effect within an arraysince it shows that the distribution and thus the probability of atailing bit is a function of baking time, and appears independent of theresistance of those cells in a previous reset state.

Distribution 830 of FIG. 8B shows the number of memory cells in thearray which have a resistance after the second baking which falls withinthe same resistance range as that after the first baking. For example,the number of cells at data point Ridx=Ridy=20 for distribution 830 arethe number of memory cells which had a resistance associated withRidx=20 after the first baking and also had a resistance associated withRidy=20 after the second baking, where Ridx=20 and Ridy=20 cover thesame resistance range as described above.

FIG. 8B also shows the predicted distribution 840 of memory cells in thearray which will have a resistance after the second baking which fallswithin the same resistance range as that after the first baking, thedistribution 840 using the probability from distribution 800 of FIG. 8A.The predicted distribution 840 further demonstrates that the resistancedistribution of the array and the probability of a memory cellexperiencing the tailing bit effect is a function of the baking time,and is independent of the resistance of the cells in a previous resetstate.

FIG. 9 is a map of the array with the dots on the map representing thelocation of the memory cells which experienced a difference in the ratioof the resistance after the first baking and second baking of greaterthan 10 (|R_(1st)/R_(2nd)|>10). As can be seen in FIG. 9, the locationof the memory cells which experience such a difference in resistance israndomly distributed throughout the array.

The erratic behavior of the tailing bit effect results in data retentionissues and bit errors for phase change memory cell arrays. ErrorCorrection Coding (ECC) may be used to address the tailing bit issues,but may result in penalties regarding write efficiency, read speed, andchip size. It is desirable therefore to provide a memory cell structureand methods for operating such structures which address these dataretention issues without ECC and result in improved data storageperformance.

FIGS. 10A and 10B illustrate schematic diagrams of memory cells havingtwo active regions arranged in series with a switching device (accessdevice) along an inter-electrode current path between first and secondelectrodes, the memory cells of FIGS. 10A and 10B addressing the tailingbit issues described above and resulting in improved data retention andreduced bit errors.

The memory cells of FIGS. 10A-10B include a switching or access device(such as a diode or transistor) 1010, first and second active regions1020, 1030, and first and second electrodes 1040, 1050. The first andsecond active regions 1020, 1030 and the switching device 1010 arearranged in series along an inter-electrode current path between firstand second electrodes 1040, 1050 so that the same electrical currentpasses through each. In operation bias circuitry (See, for example, biascircuitry voltage and current sources 1236 of FIG. 12) coupled to thefirst and second electrodes 1040, 1050 applies bias arrangements to thememory cell to store a single bit. The bias arrangements include a firstbias arrangement so that current flows along the inter-electrode pathsufficient to induce a high resistance condition (generally amorphousstate) in both the first and second active regions 1020, 1030 toestablish a high resistance state in the memory cell to store a firstvalue of the bit in the memory cell. The bias arrangements include asecond bias arrangement so that current flows along the inter-electrodepath sufficient to induce a low resistance condition (generallycrystalline state) in both the first and second active regions 1020,1030 to establish a low resistance state in the memory cell to store asecond value of the bit in the memory cell. Thus, the first and secondactive regions 1020, 1030 are both induced to a generally amorphous(high resistance) state when the memory cell is in the high resistancereset state, and both the first and second active regions 1020, 1030 areboth induced to a generally crystalline (low resistance) state when thememory cell is in a low resistance programmed state.

The high resistance state of the memory cell has a minimum resistanceindicating that at least one of the active regions 1020, 1030 is in thehigh resistance condition, and the low resistance state of the memorycell has a maximum resistance indicating that both the first and secondactive regions 1020, 1030 are in the low resistance condition.

Sense circuitry (See, for example sense circuitry 1224 of FIG. 12)coupled to the memory cell senses the value of the bit in the memorycell by determining whether the memory cell has a resistancecorresponding to the low resistance state or to the high resistancestate. The value of the bit may be determined, for example, bycomparison of current in the inter-electrode path to that of a suitablereference current by sense amplifiers of the sense circuitry.

Since the high resistance state of the memory cell has a minimumresistance indicating that at least one of the active regions 1020, 1030is in the high resistance condition, the data value stored in the memorycell can be determined even if one of the active regions 1020, 1030experiences the tailing bit effect.

In the memory cell of FIG. 10A the first active region 1020 is arrangedalong the inter-electrode current path between the switching device 1010and the second active region 1030, while in the memory cell of FIG. 10Bthe switching device 1010 is arranged along the inter-electrode currentpath between the first and second active regions 1020, 1030.

Since the active regions 1020, 1030 are both in a generally amorphousstate when the memory cell is in a reset state, by arranging the activeregions 1020, 1030 in series and spacing them apart from each other thetailing bit failure rate of an array of memory cells can besignificantly reduced (by more than 10⁴ in some instances) without theuse of ECC. For example, if the memory cell is in the reset state andone of the active regions experiences the tailing bit effect, the seriesarrangement results in the other active region “guarding” the resetstate of the memory cell. Thus, if each active region has an independentprobability P (P<1) of experiencing the tailing bit effect, arrangingthe two active regions in series reduces the probability of the memorycell experiencing the tailing bit effect to P². Additionally, it will beunderstood that the probability of a memory cell experiencing thetailing bit effect will be reduced even in instances in which there issome correlation between the tailing bit effects of the two (or more)active regions.

In the schematic diagrams of FIGS. 10A and 10B the memory cells have twoactive regions are arranged series. However, it will be understood thatthe present invention generally includes memory cells having two or moreactive regions arranged in series. For a memory cell having N (N>1)active regions arranged in series and if each active region has anindependent probability of P (P<1) of experiencing the tailing biteffect, the probability of the memory cell experiencing the tailing biteffect is reduced to P^(N).

FIG. 11A illustrates a method for programming a memory cell having aplurality of active regions arranged in series along an inter-electrodecurrent path, the method addressing the issues of the tailing bit effectand resulting in improved data retention.

At step 1100 a data value of a bit of data to be stored in a selectedmemory cell is determined. If the data value of the bit to be stored inthe memory cell is a first data value, at step 1110 all of the pluralityof active regions are set to a high resistance generally amorphouscondition to establish a high resistance state in the memory cell.Instead, if the data value of the bit to be stored in the memory cell isa second data value, at step 1120 all of the plurality of active regionsare set to a low resistance generally crystalline condition to establisha low resistance state in the memory cell.

Since all of the active regions have been set to a high resistancecondition when the high resistance state of the memory cell isestablished, the series arrangement of the active regions allows thememory cell to maintain a relatively high resistance so long as at leastone of the active regions maintains the high resistance state.

FIG. 11B illustrates a method for sensing a bit stored in a selectedmemory cell having a plurality of active regions arranged in seriesalong an inter-electrode current path. As step 1150 it is determinedwhether the resistance of the memory cell corresponds to the highresistance state or the low resistance state, the high resistance statehaving a minimum resistance indicating that at least one of the activeregions is in the high resistance condition and the low resistance statehaving a maximum resistance indicating that at least all of the activeregions are in the low resistance condition.

Thus, the method allows the data value stored to be determined even ifsome of the active regions have experienced the tailing bit effect. Ifthe resistance of the memory cell corresponds to the high resistancestate than at least one of the active regions is in the high resistancestate and the data value sensed at step 1160 is the first value of thebit. Otherwise, the resistance of the memory cell is such that all ofthe active regions are in the low resistance condition and the datavalue sensed at step 1170 is the second value.

FIG. 12 is a simplified block diagram of an integrated circuit 1210including a memory array 1212 implemented using memory cells adapted tostore one bit of data and having two or more active regions arranged inseries. A word line decoder 1214 having read, set and reset modes iscoupled to and in electrical communication with a plurality of wordlines 1216 arranged along rows in the memory array 1212. A bit line(column) decoder 1218 is in electrical communication with a plurality ofbit lines 1220 arranged along columns in the array 1212 for reading,setting, and resetting the phase change memory cells (not shown) inarray 1212. Addresses are supplied on bus 1222 to word line decoder anddrivers 1214 and bit line decoder 1218. Sense circuitry (Senseamplifiers) and data-in structures in block 1224, including voltageand/or current sources for the read, set, and reset modes are coupled tobit line decoder 1218 via data bus 1226. Data is supplied via a data-inline 1228 from input/output ports on integrated circuit 1210, or fromother data sources internal or external to integrated circuit 1210, todata-in structures in block 1224. Other circuitry 1230 may be includedon integrated circuit 1210, such as a general purpose processor orspecial purpose application circuitry, or a combination of modulesproviding system-on-a-chip functionality supported by array 1212. Datais supplied via a data-out line 1232 from the sense amplifiers in block1224 to input/output ports on integrated circuit 1210, or to other datadestinations internal or external to integrated circuit 1210.

A controller 1234 implemented in this example, using a bias arrangementstate machine, controls the application of bias circuitry voltage andcurrent sources 1236 for the application of bias arrangements includingread, program, erase, erase verify and program verify voltages and/orcurrents for the word lines and bit lines. Controller 1234 may beimplemented using special-purpose logic circuitry as known in the art.In alternative embodiments, controller 1234 comprises a general-purposeprocessor, which may be implemented on the same integrated circuit toexecute a computer program to control the operations of the device. Inyet other embodiments, a combination of special-purpose logic circuitryand a general-purpose processor may be utilized for implementation ofcontroller 1234.

As shown in FIG. 13, each of the memory cells of array 1212 includes anaccess transistor (or other access device such as a diode) and phasechange material having a plurality of active regions arranged in series.In FIG. 13 four memory cells 1330, 1332, 1334, 1336 having respectiveplurality of active regions 1340, 1342, 1344, 1346 are illustrated,representing a small section of an array that can include millions ofmemory cells. In the illustration of FIG. 13 each plurality of activeregions is shown containing two active regions, although it will beunderstood that each plurality of active regions may contain more thantwo active regions.

Sources of each of the access transistors of memory cells 1330, 1332,1334, 1336 are connected in common to source line 1354 that terminatesin a source line termination circuit 1355, such as a ground terminal. Inanother embodiment the source lines of the access devices are notelectrically connected, but independently controllable. The source linetermination circuit 1355 may include bias circuitry such as voltagesources and current sources, and decoding circuits for applying biasarrangements, other than ground, to the source line 1354 in someembodiments.

A plurality of word lines including word lines 1356, 1358 extend inparallel along a first direction. Word lines 1356, 1358 are inelectrical communication with word line decoder 1214. The gates ofaccess transistors of memory cells 1330 and 1334 are connected to wordline 1356, and the gates of access transistors of memory cells 1332 and1336 are connected in common to word line 1358.

A plurality of bit lines including bit lines 1360, 1362 extend inparallel in a second direction and are in electrical communication withbit line decoder 1218. In the illustrated embodiment each of theplurality of active regions are arranged between the drain of thecorresponding access device and the corresponding bit line.Alternatively, the active regions may be on the source side of thecorresponding access device, or on both the source and drain sides ofthe corresponding access device.

It will be understood that the memory array 1212 is not limited to thearray configuration illustrated in FIG. 13, and additional arrayconfigurations can also be used. Additionally, instead of MOStransistors bipolar transistors or diodes may be used as access devicesin some embodiments.

In operation each of the memory cells in the array 1212 store a singlebit of data depending upon the total resistance of the correspondingplurality of active regions. The data value may be determined, forexample, by comparison of current on a bit line for a selected memorycell to that of a suitable reference current by sense amplifiers ofsense circuitry 1224. The reference current can be established to that apredetermined range of currents correspond to a logical “0”, and adiffering range of current correspond to a logical “1”.

Reading or writing to a memory cell of array 1212, therefore, can beachieved by applying a suitable voltage to one of word lines 1358, 1356and coupling one of bit lines 1360, 1362 to a voltage source so thatcurrent flows through the selected memory cell. For example, a currentpath 1380 through a selected memory cell (in this example memory cell1330 and corresponding plurality of active regions 1340) is establishedby applying voltages to the bit line 1360, word line 1356, and sourceline 1354 sufficient to turn on the access transistor of memory cell1330 and induce current in path 1380 to flow from the bit line 1360 tothe source line 1354, or vice-versa. The level and duration of thevoltages applied is dependent upon the operation performed, e.g. areading operation or a writing operation.

In a reset (or erase) operation of the memory cell 1330, word linedecoder 1214 facilitates providing word line 1356 with a suitablevoltage pulse to turn on the access transistor of the memory cell 1330.Bit line decoder 1218 facilitates supplying a voltage pulse to bit line1360 of suitable amplitude and duration to induce a current to flowthough all of the plurality of active regions 1340, the current raisingthe temperature of all of the plurality of active regions 1340 above thetransition temperature of the phase change material and also above themelting temperature to place all of the plurality of active regions in aliquid state. The current is then terminated, for example by terminatingthe voltage pulses on the bit line 1360 and on the word line 1356,resulting in a relatively quick quenching time as all of active regionsin the plurality of active regions 1340 rapidly cool to stabilize to ahigh resistance generally amorphous condition and setting the memorycell to the high resistance state. The reset operation can also comprisemore than one pulse, for example using a pair of pulses.

In a set (or program) operation of the selected memory cell 1330, wordline decoder 1214 facilitates providing word line 1356 with a suitablevoltage pulse to turn on the access transistor of the memory cell 1330.Bit line decoder 1218 facilitates supplying a voltage pulse to bit line1360 of suitable amplitude and duration to induce a current to flowthrough all of the plurality of active regions 1340, the current pulsesufficient to raise the temperature of all of the active regions of theplurality of active regions 1340 above the transition temperature andcause a transition in all of the active regions of the plurality ofactive regions 1340 from the high resistance generally amorphouscondition into a low resistance generally crystalline condition, thistransition lowering the resistance of all of the plurality of activeregions and setting the memory cell to the low resistance state.

In a read (or sense) operation of the data value of the bit stored inthe memory cell 1330, word line decoder 1214 facilitates providing wordline 1356 with a suitable voltage pulse to turn on the access transistorof the memory cell 1330. Bit line decoder 1218 facilitates supplying avoltage to bit line 1360 of suitable amplitude and duration to inducecurrent to flow through all of the plurality of active regions 1340 thatdoes not result in any of the active regions in the plurality of activeregions 1340 undergoing a change in resistive state. The current on thebit line 1360 and through the plurality of active regions 1340 isdependent upon the resistance of, and therefore the data stateassociated with, the memory cell. Thus, the data state of the memorycell may be determined by detecting whether the resistance of the memorycell 1330 corresponds to the high resistance state or the low resistancestate, for example by comparison of the current on bit line 1360 with asuitable reference current by sense amplifiers of sense circuitry 1224.

FIG. 14 illustrates the measured resistance distributions 1400 and 1410of a 512 Kb array of single active region phase change memory cells withone switching device (1T1R) after baking at 130° C. for 10 hours and 22hours respectively. The dashed line 1450 represents a thresholdresistance value R_(SA) of 100 Kohm used to determine the data value ofa single bit stored in the memory cells. As can be seen in the Figure,the 1T1R distributions 1400 and 1410 include memory cells experiencingthe tailing bit effect and having a resistance below R_(SA), resultingin bit errors for those cells.

FIG. 14 also illustrates computed resistance distributions 1420 and 1430for a simulated 256 Kb array of memory cells having two active regionsarranged in series and spaced apart from one another (1T2R), the 256 Kbdistributions 1420 and 1430 computed from the measured 512 Kb 1T1R arraydistributions 1400 and 1410 respectively. The resistance of a 1T2Rmemory cell within the 256 Kb array distribution 1420 is computed bysumming the resistance of two 1T1R memory cells within the distribution1400, and the resistance of a 1T2R memory cell within the 256 Kb arraydistribution 1430 is computed by summing the resistance of two 1T1Rmemory cells within the distribution 1410.

As can be seen in FIG. 11, the additional active region of the 1T2Rmemory cells guards the reset state of the memory cells, resulting incomputed distributions 1420 and 1430 which are completely above R_(SA),thus avoiding bit errors.

FIGS. 15A and 15B further demonstrate the guarding effect the secondactive region has on memory cells in the reset state. FIG. 15A is a mapof an array of memory cells having a select device (such as a transistoror diode) and a single active region (1T1R), the dots on the map showingthe location of the 1T1R memory cells having a resistance less thanR_(SA)=100 Kohm after 50 hours of baking at 130° C. FIG. 15B shows thesimulated result of memory cells having a select device and two activeregions spaced apart and arranged in series (1T2R), the result of FIG.15B computed from the measured data of FIG. 15A by summing theresistance of two memory cells in the array of FIG. 15B. As can be seenin FIG. 15A, none of the memory cells have a computed resistance belowR_(SA) after 50 hours of baking at 130° C.

FIG. 16 illustrates a graph of the failure rate of cells in an arrayversus time for various temperatures for both 1T1R and 1T2R memory cellshaving memory material with a computed activation energy Ea=2.04 eV. Thedata points associated with dashed line 1600 represent the measuredfailure rate (the percentage of cells in a reset state with a resistancebelow a threshold value) of an array of 1T1R memory cells versus timebaked at 150° C., the dashed line 1600 being a shifted curve of dashedline 1620.

Line 1610 represents the computed failure rate of an array of 1T2Rmemory cells baked at 150° C. using the measured data of the 1T1R memorycells associated with line 1300. As can be seen in the Figure, asignificant improvement in the failure rate is achieved at 150° C. forthe tailing portion of the line 1610, indicating a reduced tailing biteffect for 1T2R memory cells.

Curves 1620 and 1630 illustrate the measured failure rate of an array of1T1R memory cells and the computed failure rate of an array of 1T2Rmemory cells baked at 130° C. respectively, again showing the improvedfailure rate performance of the 1T2R memory cells. Additionally, curves1640 and 1650 show the expected failure rate of the 1T1R array and thecomputed failure rate of the 1T2R array baked at 85° C. respectively.

FIG. 16 also includes a curve 1660 of published extrapolated failurerate data at 85° C. of a 1T1R memory cell array having a computedactivation energy of 2.4 eV using measured failure rate data at 160° C.,140° C., and 125° C. See Gleixner et. al., “Data RetentionCharacterization of Phase-Change Memory Arrays”, IEEE 45th AnnualInternational Reliability Physics Symposium, pp. 542-546, 2007, which isincorporated by reference herein. Based on the computed results above,it is expected that a 1T2R array of memory cells will further reduce thefailure rate of the 1T1R memory cells of curve 1660.

FIG. 17-22 illustrate cross-sectional views of memory cells comprisingphase change material having first and second active regions arranged inseries along an inter-electrode current path between first and secondelectrodes, which may be implemented in array of array 1212 of FIG. 12.

FIG. 17 illustrates a cross-sectional view of a first memory cell 1700having first and second active regions 1722, 1732 arranged in seriesalong an inter-electrode current path between a first electrode 1740 anda second electrode 1750.

The memory cell includes a first memory element 1720 comprising phasechange material on the first electrode 1740, and second memory element1730 comprising phase change material underlying the second electrode1750. The memory elements 1720, 1730 may each comprise, for example, oneor more materials from the group of Ge, Sb, Te, Se, In, Ti, Ga, Bi, Sn,Cu, Pd, Pb, Ag, S, Si, O, P, As, N and Au

The memory cell 1700 also includes an inner electrode 1710 contactingthe first memory element 1720 at a first contact surface 1714 andcontacting the second memory element 1730 at a second contact surface1716, the inner electrode 1710 surrounded by dielectric (not shown) andelectrically coupling the first memory element 1720 to the second memoryelement 1730.

The first and second electrodes 1740, 1750 and the inner electrode 1710may each comprise, for example, TiN or TaN. TiN may be preferred inembodiments in which the memory elements 1720, 1730 each comprise GST(discussed below) because is makes a good contact with GST, it is acommon material used in semiconductor manufacturing, and it provides agood diffusion barrier at the higher temperatures at which GSTtransitions, typically in the 600-7000 C range. Alternatively, the topand bottom electrodes 1750, 1740 and the inner electrode 1710 may beTiA1N or TaA1N, or comprises, for further examples, one or more elementsselected from the group consisting of Ti, W, Mo, Al, Ta, Cu, Pt, Ir, La,Ni, N, O, and Ru and combinations thereof. The inner electrode 1710 mayalso comprise a material having a resistivity greater than the highestresistive state of the materials of the first and second memory elements1720, 1730, the larger resistivity allowing the inner electrode 1710 toact as a heater and result in a larger temperature change in the activeregions 1722, 1732 for a given current density.

A conductive contact 1760 couples the memory cell 1700 to accesscircuitry such as a transistor or diode, the conductive contact 1760comprising a refractory metal such as tungsten in the illustratedembodiment. Other metals that could be used include Ti, Mo, Al, Ta, Cu,Pt, Ir, La, Ni, and Ru. Other contact structures and materials can beused as well. For example, the conductive contact 1760 may comprise adoped semiconductor material which is a drain or source region of anaccess transistor.

The memory cell 1700 also includes a conductive via 1770 extendingthrough dielectric (not shown) to couple the second electrode 1750 to abit line or ground for example. The conductive via 1770 may comprise,for example, any of the materials discussed above with reference to theconductive contact 1760. Alternatively, the conductive via 1770 may beomitted and the second electrode 1750 may comprise a portion of a bitline.

In operation, voltages on the conductive via 1770 and contact 1760 caninduce a current to flow along an inter-electrode current path from thefirst electrode 1740 to the second electrode 1750, or vice versa, viathe first memory element 1720, the inner electrode 1710, and the secondmemory element 1730.

The inner electrode 1710 has a width 1712 (which in some embodiments isa diameter) less than that of the first and second electrodes 1740, 1750and the first and second memory elements 1720, 1730. This difference inwidth concentrates current density in the regions of the first andsecond memory elements 1720, 1730 adjacent the inner electrode 1710,resulting in the first and second active regions 1722, 1732 adjacentrespective contact surfaces 1714, 1716 as shown in the Figure. As can beseen in FIG. 17, the active regions 1722, 1732 are spaced apart from oneanother and arranged in series. When the reset state is established inthe memory cell 1700, both of the active regions 1722 and 1732 are in ahigh resistance generally amorphous condition. Thus, the memory cell1700 has a significantly reduced probability of experiencing the tailingbit effect and has improved failure rate performance over memory cellshaving only a single active region.

Embodiments of the memory cell 1700 include phase change based memorymaterials, including chalcogenide based materials and other materials,for the memory elements 1720, 1730 respectively. Chalcogens include anyof the four elements oxygen (O), sulfur (S), selenium (Se), andtellurium (Te), forming part of group VIA of the periodic table.Chalcogenides comprise compounds of a chalcogen with a moreelectropositive element or radical. Chalcogenide alloys comprisecombinations of chalcogenides with other materials such as transitionmetals. A chalcogenide alloy usually contains one or more elements fromgroup IVA of the periodic table of elements, such as germanium (Ge) andtin (Sn). Often, chalcogenide alloys include combinations including oneor more of antimony (Sb), gallium (Ga), indium (In), and silver (Ag).Many phase change based memory materials have been described intechnical literature, including alloys of: Ga/Sb, In/Sb, In/Se, Sb/Te,Ge/Te, Ge/Sb/Te, In/Sb/Te, Ga/Se/Te, Sn/Sb/Te, In/Sb/Ge, Ag/In/Sb/Te,Ge/Sn/Sb/Te, Ge/Sb/Se/Te and Te/Ge/Sb/S. In the family of Ge/Sb/Tealloys, a wide range of alloy compositions may be workable. Thecompositions can be characterized as Te_(a)Ge_(b)Sb_(100−(a+b)). Oneresearcher has described the most useful alloys as having an averageconcentration of Te in the deposited materials well below 70%, typicallybelow about 60% and ranged in general from as low as about 23% up toabout 58% Te and most preferably about 48% to 58% Te. Concentrations ofGe were above about 5% and ranged from a low of about 8% to about 30%average in the material, remaining generally below 50%. Most preferably,concentrations of Ge ranged from about 8% to about 40%. The remainder ofthe principal constituent elements in this composition was Sb. Thesepercentages are atomic percentages that total 100% of the atoms of theconstituent elements. (Ovshinsky U.S. Pat. No. 5,687,112, cols. 10-11.)Particular alloys evaluated by another researcher include Ge₂Sb₂Te₅,GeSb₂Te₄ and GeSb₄Te₇ (Noboru Yamada, “Potential of Ge—Sb—TePhase-Change Optical Disks for High-Data-Rate Recording”, SPIE v.3109,pp. 28-37 (1997).) More generally, a transition metal such as chromium(Cr), iron (Fe), nickel (Ni), niobium (Nb), palladium (Pd), platinum(Pt) and mixtures or alloys thereof may be combined with Ge/Sb/Te toform a phase change alloy that has programmable resistive properties.Specific examples of memory materials that may be useful are given inOvshinsky '112 at columns 11-13, which examples are hereby incorporatedby reference.

Chalcogenides and other phase change materials are doped with impuritiesin some embodiments to modify conductivity, transition temperature,melting temperature, and other properties of memory elements using thedoped chalcogenides. Representative impurities used for dopingchalcogenides include nitrogen, silicon, oxygen, silicon dioxide,silicon nitride, copper, silver, gold, aluminum, aluminum oxide,tantalum, tantalum oxide, tantalum nitride, titanium and titanium oxide.See, e.g., U.S. Pat. No. 6,800,504, and U.S. Patent ApplicationPublication No. U.S. 2005/0029502.

Phase change alloys are capable of being switched between a firststructural state in which the material is in a generally amorphous solidphase, and a second structural state in which the material is in agenerally crystalline solid phase in its local order in the activechannel region of the cell. These alloys are at least bistable. The termamorphous is used to refer to a relatively less ordered structure, moredisordered than a single crystal, which has the detectablecharacteristics such as higher electrical resistivity than thecrystalline phase. The term crystalline is used to refer to a relativelymore ordered structure, more ordered than in an amorphous structure,which has detectable characteristics such as lower electricalresistivity than the amorphous phase. Typically, phase change materialsmay be electrically switched between different detectable states oflocal order across the spectrum between completely amorphous andcompletely crystalline states. Other material characteristics affectedby the change between amorphous and crystalline phases include atomicorder, free electron density and activation energy. The material may beswitched either into different solid phases or into mixtures of two ormore solid phases, providing a gray scale between completely amorphousand completely crystalline states. The electrical properties in thematerial may vary accordingly.

Phase change alloys can be changed from one phase state to another byapplication of electrical pulses. It has been observed that a shorter,higher amplitude pulse tends to change the phase change material to agenerally amorphous state. A longer, lower amplitude pulse tends tochange the phase change material to a generally crystalline state. Theenergy in a shorter, higher amplitude pulse is high enough to allow forbonds of the crystalline structure to be broken and short enough toprevent the atoms from realigning into a crystalline state. Appropriateprofiles for pulses can be determined, without undue experimentation,specifically adapted to a particular phase change alloy. In followingsections of the disclosure, the phase change material is referred to asGST, and it will be understood that other types of phase changematerials can be used. A material useful for implementation of a PCRAMdescribed herein is Ge₂Sb₂Te₅.

Other programmable resistive memory materials may be used in otherembodiments of the invention, including N₂ doped GST, Ge_(x)Sb_(y), orother material that uses different crystal phase changes to determineresistance; Pr_(x)Ca_(y)MnO₃, Pr_(x)Sr_(y)MnO₃, ZrO_(x), or othermaterial that uses an electrical pulse to change the resistance state;7,7,8,8-tetracyanoquinodimethane (TCNQ), methanofullerene 6,6-phenylC61-butyric acid methyl ester (PCBM), TCNQ-PCBM, Cu-TCNQ, Ag-TCNQ,C60-TCNQ, TCNQ doped with other metal, or any other polymer materialthat has a bistable or multi-stable resistance state controlled by anelectrical pulse.

An exemplary method for forming chalcogenide material usesPVD-sputtering or magnetron-sputtering method with source gas(es) of Ar,N2, and/or He, etc. at the pressure of 1 mTorr˜100 mTorr. The depositionis usually done at room temperature. A collimator with an aspect ratioof 1˜5 can be used to improve the fill-in performance. To improve thefill-in performance, a DC bias of several tens of volts to severalhundreds of volts is also used. On the other hand, the combination of DCbias and the collimater can be used simultaneously.

A post-deposition annealing treatment in a vacuum or in an N₂ ambient isoptionally performed to improve the crystallize state of chalcogenidematerial. The annealing temperature typically ranges from 100° C. to400° C. with an anneal time of less than 30 minutes.

The thickness of chalcogenide material depends on the design of cellstructure. In general, a chalcogenide material with thickness of higherthan 8 nm can have a phase change characterization so that the materialexhibits at least two stable resistance states.

FIGS. 18-22 also illustrate memory cells having first and second regionsarranged in series. As will be understood, the materials described abovewith reference to elements of the memory cell of FIG. 17 may beimplemented in the memory cells of FIGS. 18-22, and thus a detaileddescription of these materials is not repeated.

FIG. 18 illustrates a cross-sectional view of a second memory cell 1800having first and second active regions 1822, 1832 arranged in seriesalong an inter-electrode current path between a first electrode 1840 anda second electrode 1850.

The memory cell 1800 includes a first memory element 1820 comprisingphase change material on the first electrode 1840, and a second memoryelement 1830 underlying the second electrode 1850.

The memory cell 1800 also includes an inner electrode 1810 contactingthe first memory element 1820 at a first contact surface 1814 andcontacting the second electrode at a second contact surface 1816. Thefirst and second memory elements 1820, 1830 and the inner electrode 1810form a multi-layer pillar surrounded by dielectric (not shown), themulti-layer pillar electrically coupling the first and second electrodes1840, 1850.

A conductive contact 1860 couples the memory cell 1800 to accesscircuitry such as a transistor or diode. Other contact structures can beused as well. For example, the conductive contact 1860 may comprise adoped semiconductor material which is a drain or source region of anaccess transistor.

The memory cell 1800 also includes a conductive via 1870 extendingthrough dielectric (not shown) to couple the second electrode 1850 to abit line or ground for example. Alternatively, the conductive via 1870may be omitted and the second electrode 1850 may comprise a portion of abit line.

In operation, voltages on the conductive via 1870 and contact 1860 caninduce a current to flow along an inter-electrode current path from thefirst electrode 1840 to the second electrode 1850, or vice versa, viathe first memory element 1820, the inner electrode 1810, and the secondmemory element 1830.

The inner electrode 1810 has a width 1812 (which in some embodiments isa diameter) substantially the same as that of the first and secondmemory elements. As used herein, the term “substantially” is intended toaccommodate manufacturing tolerances. The inner electrode 1810 also hasa width less than that of the first and second electrodes 1840, 1850,resulting in the current density in operation to be concentrated in themulti-layer pillar. In some embodiments the inner electrode 1810comprises a heater material, resulting in the active regions 1822, 1832adjacent the respective contact surfaces 1814, 1816.

FIG. 19 illustrates a cross-sectional view of a third memory cell 1900having first and second active regions 1922, 1932 arranged in seriesalong an inter-electrode current path between a first electrode 1940 anda second electrode 1950.

The memory cell 1900 includes a memory element 1920 comprising phasechange material electrically coupling the first electrode 1940 and thesecond electrode 1950. The first electrode 1940 contacts the memoryelement 1920 at first contact surface 1944, and the second electrode1950 contacts the memory element 1920 at a second contact surface 1954.

The memory element 1920 has a width 1926 greater than the width 1942 ofthe first electrode 1940 and greater than the width 1952 of the topelectrode 1950. This difference in widths concentrates current densityin the regions of memory element 1920 adjacent the first and secondelectrodes 1940, 1950. Additionally, the memory element 1920 has athickness 1928 sufficient to separate the generally amorphous activeregions 1920, 1930 when the memory cell 1900 is in the reset state.

A conductive contact 1960 couples the memory cell 1900 to accesscircuitry such as a transistor or diode. Other contact structures can beused as well. For example, the conductive contact 1960 may comprise adoped semiconductor material which is a drain or source region of anaccess transistor.

The memory cell 1900 also includes a conductive via 1970 extendingthrough dielectric (not shown) to couple the second electrode 1950 to abit line or ground for example. Alternatively, the conductive via 1970may be omitted and the second electrode 1950 may comprise a portion of abit line.

In operation, voltages on the conductive via 1970 and contact 1960 caninduce a current to flow along an inter-electrode current path from thefirst electrode 1940 to the second electrode 1950, or vice versa, viathe memory element 1920.

FIG. 20 illustrates a cross-sectional view of a fourth memory cell 2000having first and second active regions 2022, 2032 arranged in seriesalong an inter-electrode current path between a first electrode 2040 anda second electrode 2050.

The memory cell 2000 includes first memory element 2020 contacting firstelectrode 2040 at first contact surface 2044, and a conductive electrode2070 on the memory element 2020. The first electrode 2040 has a width2042 less than that of the first memory element 2020 and the electrode2070. This difference in width concentrates the current density in theportion of the memory element 2020 adjacent the first electrode 2040,resulting in the active region 2022 adjacent the first contact surface2044 as shown.

An inner electrode 2010 contacts second memory element 2030 at secondcontact surface 2034 and couples the second memory element 2030 to theelectrode 2070. The inner electrode 2010 has a width 2012 less than thatof the second memory element 2030 and second electrode 2050. Thisdifference in width concentrates the current density in the portion ofthe second memory element 2030 adjacent the inner electrode 2010,resulting in the active region 2032 adjacent the second contact surface2034 as shown.

A conductive contact 2060 couples the memory cell 2000 to accesscircuitry such as a transistor or diode. Other contact structures can beused as well. For example, the conductive contact 2060 may comprise adoped semiconductor material which is a drain or source region of anaccess transistor.

The second electrode 2050 is coupled to a bit line or ground by aconductive via for example. Alternatively, the second electrode 1850 maycomprise a portion of a bit line.

In the memory cells of FIGS. 17-20 the active regions are on differentplanes such that one active region overlies the other active region.FIGS. 21-22 illustrate memory cells in which the first and second activeregions 1020, 1030 are physically spaced apart from each other butlocated within the same physical plane.

FIG. 21 illustrates a cross-sectional view of a fifth memory cell 2100having first and second active regions 2122, 2132 arranged in seriesalong an inter-electrode current path between a first electrode 2140 aand a second electrode 2150 b.

Memory cell 2100 includes first and second portions 2102 a and 2102 bphysically located in the same plane and electrically coupled to eachother as represented by line 2180. The electrical coupling representedby line 2180 can be achieved using various techniques such as conductivelines and/or contacts

The first and second portions 2102 a and 2108 b are substantially thesame, where the term “substantially” is intended to accommodatemanufacturing tolerances. Each portion 2102 of the memory cell 2100includes a conductive contact 2160, a bottom electrode 2140 on theconductive contact 2160, a memory element 2120 on the bottom electrode2140, a top electrode 2150 on the memory element 2120, and a conductivevia 2170 on the top electrode 2150.

The bottom electrodes 2140 have a width less than that of the memoryelements 2120 and the top electrodes 2150. In operation this differencein width concentrates the current density to the portion of the memoryelements 2120 adjacent the bottom electrode 2140, resulting in activeregion 2122 within memory element 2120 a and active region 2132 withinmemory element 2120 b as shown in the Figure.

FIG. 22 illustrates a cross-sectional view of a sixth memory cell havingfirst and second active regions 2222, 2232 arranged in series along aninter-electrode current path between a first electrode 2240 a and asecond electrode 2250 b.

Memory cell 2200 includes first and second portions 2202 a and 2202 bphysically located in the same plane, the first portion 2202 a coupledto the source of a transistor and the second portion 2202 b coupled tothe drain of the transistor.

The first and second portions 2202 a and 2202 b are substantially thesame, where the term “substantially” is intended to accommodatemanufacturing tolerances. Each portion 2202 of the memory cell 2200includes a conductive contact 2260, a bottom electrode 2240 on theconductive contact 2260, a memory element 2220 on the bottom electrode2240, a top electrode 2250 on the memory element 2220, and a conductivevia 2270 on the top electrode 2250.

The conductive contact 2260 a is coupled to doped region 2280 within asemiconductor substrate 2290, doped region 2280 acting as the source ofa transistor select device. The conductive contact 2260 b is coupled todoped region 2282 within semiconductor substrate 2290, doped region 2282acting as the drain region of the transistor. The transistor alsoincludes a gate 2292 (which may comprise a portion of a word line)overlying the substrate 2290.

The conductive via 2270 a is coupled to a first bit line, and theconductive via 1970 b is coupled to a second bit line.

In operation, voltages on the gate 2292 and the first and second bitlines can induce a current from the second portion 2202 b to the firstportion 2202 a, or vice versa, via the transistor. The bottom electrodes2240 have a width less than that of the memory elements 2220 and the topelectrodes 2250. This difference in width concentrates the currentdensity to the portion of the memory elements 2220 adjacent the bottomelectrode 2240, resulting in active region 2222 within memory element2220 a and active region 2232 within memory element 2220 b as shown inthe Figure.

The memory cells illustrated in FIGS. 17-22 include two active regionswhich are in a high resistance generally amorphous condition when thehigh resistance state is established in the memory cell, thus addressingthe issues of tailing bits and resulting in reduced failure rate formemory cell arrays. However, it will be understood that the presentinvention also includes memory cells having more than two active regionsarranged in series and each spaced apart from each other, all of themore than two active regions in a high resistance generally amorphouscondition when the high resistance reset state is established in thememory cell. For example, the structures illustrated in FIGS. 17-22 maybe serially repeated and/or combined in series to obtain more than twoactive regions arranged in series.

Generally, the two or more active regions of memory cells describedherein may be electrically arranged in series and coupled to a terminalof a select device such as a field effect transistor, bipolar junctiontransistor, or diode (for example a drain or source of a selecttransistor), and/or may be coupled to different terminals of the selectdevice (for example one or more active regions coupled to the drain andone or more active regions coupled to the source).

Also, the two or more active regions of memory cells described hereinmay be physically arranged in the same plane and electrically coupled toeach other in series, and/or may be arranged in different planes andelectrically coupled to each other in series.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention.

1. A memory device comprising: a memory cell comprising a firstelectrode, a second electrode, and phase change material having firstand second active regions arranged in series along an inter-electrodecurrent path between the first electrode and the second electrode; biascircuitry adapted to apply bias arrangements to the memory cell to storea bit, the bias arrangements including: a first bias arrangement adaptedto establish a high resistance state in the memory cell by inducing ahigh resistance condition in both the first and second active regions tostore a first value of the bit in the memory cell, the high resistancestate having a minimum resistance indicating that at least one of theactive regions is in the high resistance condition, and a second biasarrangement adapted to establish a low resistance state in the memorycell by inducing a low resistance condition in both the first and secondactive regions to store a second value of the bit in the memory cell,the low resistance state having a maximum resistance indicating thatboth the first and second active regions are in the low resistancecondition; and sense circuitry to sense the value of the bit in thememory cell by determining whether the memory cell has a resistancecorresponding to the low resistance state or to the high resistancestate.
 2. The device of claim 1, wherein the memory cell furthercomprises a diode having a first terminal and a second terminal, thefirst active region coupled to the first terminal and the second activeregion coupled to the second terminal.
 3. The device of claim 1, whereinthe memory cell further comprises a transistor having a drain terminaland a source terminal, the first active region coupled to the drainterminal and the second active region coupled to the source terminal. 4.The device of claim 1, wherein the memory cell further comprises abipolar junction transistor having an emitter terminal and a collectorterminal, the first active region coupled to the emitter terminal andthe second active region coupled to the collector terminal.
 5. Thedevice of claim 1, wherein the memory cell further comprises a bipolarjunction transistor having an emitter terminal and a collector terminal,the first active region coupled to the emitter terminal, and the secondactive region coupled to the emitter terminal via the first activeregion.
 6. The device of claim 1, wherein the memory cell furthercomprises a bipolar junction transistor having an emitter terminal and acollector terminal, the first active region coupled to the collectorterminal, and the second active region coupled to the collector terminalvia the first active region.
 7. The device of claim 1, wherein thememory cell further comprises a transistor having a drain terminal and asource terminal, the second active region coupled to the source terminalvia the first active region.
 8. The device of claim 1, wherein thememory cell further comprises a transistor having a drain terminal and asource terminal, the second active region coupled to the drain terminalvia the first active region.
 9. The device of claim 1, wherein: thememory cell further comprises a memory element comprising the phasechange material, the first and second active regions within the memoryelement; the first electrode contacting the memory element at a firstcontact surface, the first electrode having a width less than that ofthe memory element such that the first active region is adjacent thefirst contact surface; and the second electrode contacting the memoryelement at a second contact surface, the second electrode having a widthless than that of the memory element such that the second active regionis adjacent the second contact surface.
 10. The device of claim 1,wherein the memory cell further comprises a first memory elementcomprising a portion of the phase change material and a second memoryelement comprising a portion of the phase change material, the firstactive region within the first memory element and the second activeregion within the second memory element.
 11. The device of claim 10,wherein: the first memory element contacts the first electrode; thesecond memory element contacts the second electrode; and the memory cellfurther comprises an inner electrode arranged along the inter-electrodecurrent path between the first and second memory elements.
 12. Thedevice of claim 11, wherein: the inner electrode contacts the firstmemory element at a first contact surface and contacts the second memoryelement at a second contact surface; the first memory element has awidth greater than that of the inner electrode such that the firstactive region is adjacent the first contact surface; and the secondmemory element has a width greater than that of the inner electrode suchthat the second active region is adjacent the second contact surface.13. The device of claim 11, wherein: the first electrode contacts thefirst memory element at a first contact surface, the first electrodehaving a width less than that of the first memory element such that thefirst active region is adjacent the first contact surface; the memorycell further comprises a conductive electrode on the first memoryelement, the conductive electrode having a width greater than that ofthe first electrode; the inner electrode on the conductive electrode andcontacting the second memory element at a second contact surface, theinner electrode having a width less than that of the conductiveelectrode such that the second active region is adjacent the secondcontact surface.
 14. The device of claim 1, wherein the first and secondactive regions are located in the same plane.
 15. The device of claim 1,wherein the first active region overlies the second active region. 16.The device of claim 1, wherein the memory cell further comprises a diodehaving a first terminal and a second terminal, the first active regioncoupled to the first terminal via the second active region.
 17. A methodfor operating a memory cell comprising a first electrode, a secondelectrode, and phase change material having first and second activeregions arranged in series along an inter-electrode current path betweenthe first electrode and the second electrode, the method comprising:determining a data value of a bit to be stored in the memory cell;applying a first bias arrangement to the memory cell if the data valueis a first value, the first bias arrangement adapted to establish a highresistance state in the memory cell by inducing a high resistancecondition in both the first and second active regions to store the firstvalue of the bit, the high resistance state having a minimum resistanceindicating that at least one of the active regions is in the highresistance condition; applying a second bias arrangement to the memorycell if the data value is a second value, the second bias arrangementadapted to establish a low resistance state in the memory cell byinducing a low resistance condition in both the first and second activeregions to store the second value of the bit, the low resistance statehaving a maximum resistance indicating that both the first and secondactive regions are in the low resistance state; and determining the datavalue of the bit stored in the memory cell by determining whether thememory cell has a resistance corresponding to the low resistance stateor to the high resistance state.
 18. The method of claim 17, wherein thedetermining the data value of the bit stored in the memory cellcomprises applying a third bias arrangement to the memory cell anddetecting a current in the memory cell, the current in the memory cellcorresponding to the data value of the bit stored in the memory cell.19. The method of claim 17, wherein the memory cell further comprises adiode having a first terminal and a second terminal, the first activeregion coupled to the first terminal and the second active regioncoupled to the second terminal.
 20. The method of claim 17, wherein thememory cell further comprises a transistor having a drain terminal and asource terminal, the first active region coupled to the drain terminaland the second active region coupled to the source terminal.
 21. Themethod of claim 17, wherein the memory cell further comprises a bipolarjunction transistor having an emitter terminal and a collector terminal,the first active region coupled to the emitter terminal and the secondactive region coupled to the collector terminal.
 22. The method of claim17, wherein the memory cell further comprises a transistor having adrain terminal and a source terminal, the second active region coupledto the source terminal via the first active region.
 23. The method ofclaim 17, wherein the memory cell further comprises a transistor havinga drain terminal and a source terminal, the first active region coupledto the source terminal and the second active region coupled to thesource terminal.
 24. The method of claim 17, wherein the memory cellfurther comprises a bipolar junction transistor having an emitterterminal and a collector terminal, the first active region coupled tothe emitter terminal, and the second active region coupled to theemitter terminal via the first active region.
 25. The method of claim17, wherein the memory cell further comprises a bipolar junctiontransistor having an emitter terminal and a collector terminal, thefirst active region coupled to the collector terminal, and the secondactive region coupled to the collector terminal via the first activeregion.
 26. The method of claim 17, wherein the memory cell furthercomprises a diode having a first terminal and a second terminal, thefirst active region coupled to the first terminal via the second activeregion.